Conventionally, microporous films have been used as separators or other materials used in cells and electrolytic capacitors. Demand for applications in lithium-ion secondary cells in particular has been increasing in recent years, and there exists a great need for high-speed production of separators.
Electrolytic fluids, positive and negative electrode active materials, and other agents are used in lithium-ion secondary cells. Therefore, a polyolefin-based polymer, particularly polyethylene or polypropylene, which is inexpensive, is typically used as the material for the separator, taking chemical resistance into account. Characteristics such as electrode short-circuit prevention, high ion permeability, ease of assembly and processing during winding of the cell, cell safety, and reliability have conventionally been desired as basic functions of separators applied in lithium-ion secondary cells and other cells employing a non-aqueous electrolyte. Uniform quality has also been greatly needed in recent years as the size, energy density, and output of cells have increased.
Well-known methods for manufacturing polyolefin microporous films involve melt-kneading a composition composed of a polyolefin resin and a plasticizer; extruding the composition from a die and molding the composition into a film form; after extrusion, stretching the film uniaxially or biaxially; after stretching, using a solvent to extract the plasticizer from the film; drying the solvent; and, as necessary after drying, further stretching or heat-treating (Japanese Laid-Open Patent Application No 5-156058, Japanese Laid-Open Patent Application No. 11-60789, Japanese Laid-Open Patent Application 2010-235707).
A well-known method for drying of the solvent after extraction of the plasticizer involves drying on drying rolls such that the width of the film is mechanically restrained, and the speed of the drying rolls is 35 to 50 m/min (Japanese Laid-Open Patent Application 2011-42805).